change appearance if polymorphing while mimicing

As per <Someone>:
> I had a game today where I was polymorphed (by a sink) into a mimic, and
> #monster-ed (hid).  The symbol on the map for me was ].  Then, I polymorphed
> again, this time into a kobold lord -- but the symbol remained ].  This
> seems wrong.
Handle this similar to the polyman code.

doc/fixes34.1

@@ -107,6 +107,7 @@ give more specific message when forbidden role attempts to use twoweapon mode
 avoid double billing if #loot causes a shop's bag of holding to explode
 when polymorphed, player killing a paper or straw golem via fire damage
 	would kill the golem twice, resulting in an impossible error
+usually stop mimicing if you polymorph while using #monster mimic capability
 
 
 Platform- and/or Interface-Specific Fixes

src/polyself.c

@@ -356,6 +356,9 @@ int	mntmp;
 		youmonst.mappearance == GOLD_PIECE) {
 	    /* stop mimicking gold immediately */
 	    if (multi < 0) unmul("");
+	} else if (mons[mntmp].mlet != S_MIMIC) {
+	    /* as in polyman() */
+	    youmonst.m_ap_type = M_AP_NOTHING;
 	}
 	if (is_male(&mons[mntmp])) {
 		if(flags.female) dochange = TRUE;